Mode suppression means for a clover-leaf slow wave circuit

ABSTRACT

A microwave velocity modulation electron tube employing a cloverleaf slow wave structure has a mode suppression cavity resonator provided in the conductive nose portions of the slow wave circuit. The mode suppression cavities are coupled through a pair of elongated coupling slots provided in the sidewalls of the noses. The mode suppression cavities are coated with a lossy wave energy attenuating material for absorbing wave energy coupled thereto. The mode suppression cavities are capacitively loaded by post structures and stagger tuned for suppressing undesired modes of oscillations.

United States Patent 3,221,205 1 1/1965 Sensider Inventor Charles E. Blinn Sunnyvale, Calif.

App]. No. 872,887

Filed Oct. 31, 1969 Patented July 20, 1971 Assignee Varian Associates Palo Alto, can.

MODE SUPPRESSION MEANS FOR A CLOVER- LEAF SDOW WAVE CIRCUIT 6 Claims, 4 Drawing Figs.

US. Cl 315/15, 315/36, 333/31 A Int. Cl 1101] 25/34 Fidt'lolsenrcll 31513.5, 3.6, 39.3;333/31 A References Cited UNITED STATES PATENTS PULSER il /I'l 3,354,347 11/1967 Hant 315/35 3,360,679 12/1967 Rubert 315/3.5 3,365,607 1/1968 Ruetz et 315/35 3,412,279 11/1968 Allen et a1. 315/35 3,454,817 7/1969 Shively et a1 3 l5/3.5

Primary ExaminerH. K. Saalbach Assistant Examiner-Saxficld Chatmon, Jr. Att0rneysStanley Z. Cole and Gerald L. Moore ABSTRACT: A microwave velocity modulation electron tube employing a cloverleaf slow wave structure has a mode suppression cavity resonator provided in the conductive nose portions of the slow wave circuit. The mode suppression cavities are coupled through a pair of elongated coupling slots provided in the sidewalls of the noses. The 'mode suppression cavities are coated with a lossy wave energy attenuating material for absorbing wave energy coupled thereto. The

mode suppression cavities are capacitively loaded by post structures and stagger tuned for suppressing undesired modes of oscillations.

PATENTEU M20197! 35941605 BAND EDGE OSCILL CHARLES E. B 4 BY Mm MODE SUPPRESSION MEANS FOR A CLOVER-LEAF SLOW WAVE CIRCUIT DESCRIPTION OF THE PRIOR ART Heretofore, cloverleaf circuits for velocity modulation tubes have employed mode suppression cavities coupled to the slow wave circuit via elongated coupling irises provided in the sidewalls of the slow wave circuit. The mode suppression cavities were one-half wavelength long, at the 11 point, curving around the outer periphery of the cloverleaf circuit with a decreasing height in the E plane and were coated on their interior walls with lossy material for absorbing wave energy coupled thereto. The mode suppression cavities operated like a high pass filter to a load and effectively suppressed band edge oscillations and other higher frequency modes, such as slot modes. However, they tended to substantially increase the diameter of the cloverleaf circuit, thus, requiring a larger electromagnet for focusing the beam than would otherwise be required for a noncavity loaded cloverleaf circuit operating at the same frequency. Such a prior art mode suppression cavity structure is disclosed and claimed in U.S. Pat. No. 3,454,817

.issued July 8, 1969 and assigned to the same assignee as the present invention.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of improved mode suppression means for a coupled cavity slow wave circuit.

One feature of the present invention is the provision in a coupled cavity slow wave circuit of the type employing conductive nose portions projecting into the coupled cavities, of mode suppression cavities resonant at or near the frequency of undesired modes of the slow wave circuit and at least partially disposed in the nose portions and coupled to the fields of the slow wave circuit via elongated coupling irises provided in the sidewalls of the nose portions of the slow wave circuit, whereby such undesiredresonant modes are effectively suppressed without substantially increasing the diameter of the slow wave circuit.

Another feature of the present invention is the same as the preceding feature wherein the mode suppression cavities are capacitively loaded for reducing their physical size such that they may be readily accommodated within the nose portions of the slow wave circuit.

Another feature of the present invention is the same as any one or more of the preceding features wherein the mode suppression cavities are tuned for resonance near the 1r mode of the slow wave circuit for suppressing the undesired upper band edge oscillation of the slow wave circuit.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal sectional schematic line diagram of a velocity modulation electron tube incorporating features of the present invention,

FIG. 2 is an enlarged sectional view of a physical realization of the structure of FIG. 1 taken along line 2-2 in the direction of the arrows,

FIG. 3 is a sectional view of the structure of FIG. 2 taken along line 33 in the direction of the arrows, and

FIG. 4 is an vs 3 diagram depicting the dispersion characteristics of the cloverleaf slow wave circuit depicted in FIGS. 13 and showing the staggered tuning of the mode suppression cavities for suppressing band edge oscillations.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. I, there is shown in schematic line diagram form, a microwave velocity modulation electron tube incorporating features of the present invention. Tube 1 is of to the same assignee as the present invention. Briefly, the tube 1 includes an electron gun assembly 2 disposed at one end of the tube for forming and projecting a beam of electrons over an elongated beam path 3 to a beam collector structure 4 disposed at the terminal end of the beam path 3. The beam collector 4 collects the beam and dissipates the energy thereof. An electromagnetic interaction circuit 5 is disposed along the beam path intermediate the electron gun 2 and collector 4 for electromagnetic interaction with the beam to produce an amplified output signal extracted from the circuit 5 via output waveguide 6. A solenoid, not shown, coaxially surrounds the interaction circuit for focusing the beam.

The microwave circuit 5 includes a plurality of reentrant driver cavity resonators 7 successively arranged along the beam path for velocity modulating the beam with r.f. wave energy to be amplified which is fed into the upstream cavity 7' via an input waveguide assembly 8 having a wave permeable gastight window assembly 9 mounted therein. The driver cavity resonators 7 are stagger tuned over a band of frequencies to obtain broadband operation. The modulated beam is fed into a cloverleaf slow wave circuit 11 disposed downstream of the cavities 7 for supplementing the gain obtained by the driver cavities 7 and for extracting the amplified output signal from the beam.

The cloverleaf slow wave circuit 11 includes a plurality of negative mutually inductively coupled cavity resonators successively arranged along the beam path. The upstream coupled cavity is terminated in a resistive load 12 to prevent undesired retlection of wave energy which could otherwise result in setting up of undesired oscillations in the slow wave circuit 11. The gun 2, interaction circuit 5, and collector 4 are enclosed in an evacuated envelope structure 13 which is completed in the output waveguide section 6 via a wave permeable microwave window 14 sealed across the waveguide 6.

The interaction circuit 5, collector 4 and the anode of the gun 2 are typically operated at ground potential, whereas the thermionic cathode 15 of the gun is pulsed negative with respect to the anode, interaction circuit 5 and collector 4 via a cathode pulser 16, such negative voltage being derived from a power supply 17 having its positive terminal grounded. Thus, the cathode emitter 15 is pulsed from ground potential to on the order of -140 kv. relative to ground via pulser 16 to initiate operation of the tube and to establish the beam voltage. In a typical example, the tube I produces 5 megawatts of peak output power at C band over a bandwidth of 10 percent.

Referring now to FIGS. 2 and 3, there is shown the physical realization of the cloverleaf slow wave circuit 11 incorporating features of the present invention. The cloverleaf circuit 11 includes a plurality of. coupled cavity resonators 21 successively arranged along the beam path 3 for electromagnetic interaction with the beam. The common wall 24 between adjacent resonators 21 includes an array of eight radially directed elongated coupling slots 22 circumferentially spaced with relation to four radially directed inwardly projecting conductive nose portions 23. The end walls 24 of the resonators 21 are centrally apertured at 25 to provide openings to accommodate passage of the beam 3 therethrough. The nose portions 23 are rotationally displaced in successive adjacent cavities by 45 in order to obtain negative mutually inductive coupling through the coupling slots 22.

The resultant cloverleaf circuit 11 has a dispersion characteristic as shown in FIG. 4. The dispersion characteristic is characterized by a low frequency resonance mode of operation to, associated with zero phase shift in the electric field between adjacent cavities and an upper cutoff resonant frequency to, associated with or 1r phase shift of the electric field in adjacent cavities. An operating band, typically of percent to percent bandwidth, is obtained intermediate in, and m, as indicated at 26 in FIG. 4. As the beam voltage is applied it increases from zero volts to the operating voltage as indicated by arrow V As the beam voltage V increases, it passes through the 11' mode resonance and heretofore has excited certain undesired band edge oscillations of the circuit 1 1 above the upper frequency of the operating band 26.

These undesired band edge oscillations are effectively sup pressed in the tube 1 of the present invention by the provision of the four mode suppression cavities 27 located at least partially within the nose portions 23 of each coupled cavity 21 of the cloverleaf slow wave circuit 11 The mode suppression cavities 27 are coupled to the fields of the slow wave circuit via the intermediary of a pair of elongated coupling slots 28 communicating through the sidewalls of the nose portions 23 to provide wave energy communication between the coupled cavity 21 and the mode suppression cavities 27.

The mode suppression cavities 27 are tuned for a resonance near the 1r mode resonant frequency of the 'slow wave circuit 11 and the interior walls of the mode suppression cavities 27 are coated with a wave energy attenuating material, such as an alloy consisting of 5.5 percent Al, 22 percent Cr, 1/2 percent Cobalt and the balance iron, for attenuating wave energy coupled from the slow wave circuit 11 into the mode suppression cavities 27.

The mode suppression cavities 27 each include a capacitive loading post structure 29 for capacitively loading the resonator 27 to decrease its physical dimensions such that it may be readily accommodated within the envelope 13. The capacitive loading posts 29, as of copper, project from one broad wall of the mode suppression cavities 27 toward the opposed wall to provide a capacitive gap 31 in a space between the free end of the post and the opposed wall. Adjustment of the size of the gap 31 provides means for tuning the resonant frequencies of the cavities 27.

Mode suppression cavities 27 are tuned to slightly different frequencies spread over the band of band edge oscillations. In other words, each of the four mode suppression cavities which is coupled to each ofthe coupled cavities 21 of the slow wave circuit is tuned to a slightly different frequency spread over the band of undesired. band edge oscillations. In a particular embodiment shown, the circuit is designed for C band operation and the capacitive posts 29 are preferred in such a circuit operating at this frequency. However, for circuits operating at other frequencies, such as frequencies higher than C band, the post structures 29 may not be necessary as the cavities may have sufficiently small size to be readily incorporated in the envelope 13 without reducing their physical size by capacitive loading. The coupling slots 28 are elongated in the radial direction, i.e., in a direction perpendicular to the longitudinal axis of the beam 3 and preferably have a length which is near a half wavelength at the center frequency of the undesired band edge oscillations to provide heavy coupling to the undesired modes of the cavities 21.

Positioning the mode suppression cavities 27 in the nose portions 23 of the circuit greatly enhances the attenuating effect of the mode suppression cavities since this positions the coupling slots 28 in regions of more intense magnetic field for modes having resonant frequencies near the upper band edge frequency w, This attenuation is obtained without increasing the diameter of the slow wave circuit. In addition, by positioning mode suppression cavities in the nose portions, two elongated coupling slots 28 are possible for coupling wave energy into each of the mode suppression cavities 27. This greatly facilitates the desired heavy coupling between the fields of the slowwave circuit cavities 21 and the mode suppression cavities 27. Mode suppression cavities 27 are provided in each of the nose sections for each of the cavities 21 of the slow wave circuit 11. The mode suppression cavities 27 may be tuned to any undesired mode of oscillation present in the slow wave circuit for suppressing such undesired mode. Examples of such undesired modes include the SH modes and slot modes.

ln additlon, the coupled cavities 2] of the slow wave circuit 11 are preferably coated with lossy attenuating material, such as an alloy consisting of 5.5 percent Al, 22 percent Cr., Cr, percent Cobalt, and the balance iron, for suppressing certain other undesired modes of oscillation in the conventional manner.

' Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What l claim is:

1. In a microwave velocity modulation electron tube, means for forming and projecting a beam of electrons over an elongated beam path, means at the terminal end of the beam path for collecting and dissipating the energy of the beam, slow wave circuit means for electromagnetic interaction with the beam, said slow wave circuit including a plurality of cavity resonators successively arranged along the beam path, negative mutual inductive coupling means communicating between said adjacent cavity resonators, the walls defining said cavities including a plurality of conductive nose portions protruding into each of said cavity resonators, mode suppression cavity resonator means coupled to the fields of said first mentioned coupled cavities of said slow wave circuit for suppressing undesired modes of oscillation associated with said slow wave circuit, THE IMPROVEMENT COMPRISING, slot coupling means in said nose portions of said slow wave circuit for coupling said mode suppression cavities to the fields of undesired modes of oscillation within said slow wave circuit, and wherein at least a portion of each of said mode suppression cavities is disposed within said nose portions of said slow wave circuit.

2. The apparatus of claim 1 wherein said slot coupling means in said nose portions includes a pair of coupling slots on opposite sides of each nose portion, said pair of slots being elongated in a direction generally perpendicular to the axis of the electron beam.

3. The apparatus of claim 1 including a lumped capacitive loading means in each of said mode suppression cavities for capacitively loading said mode suppression cavities to reduce the physical size thereof for a given resonant frequency.

4. The apparatus of claim 1 wherein said mode suppression cavities are tuned for resonance near the 1r mode resonant frequency of said slow wave circuit for suppressing undesired upper band edge oscillations of said slow wave circuit.

5. The apparatus of claim 4 wherein there are four mode suppression cavities coupled to each of said first mentioned coupled cavities of said slow wave circuit, and wherein each of said four mode suppression cavities is tuned to a different frequency within a band of frequencies centered near the 11' mode resonant frequency of said slow wave circuit and above the operating band of said slow wave circuit.

6. The apparatus of claim 1 including wave energy attenuating coating means coating the interior surfaces of each of said mode suppression cavities for attentuating wave energy coupled into said mode suppression cavities. 

1. In a microwave velocity modulation electron tube, means for forming and projecting a beam of electrons over an elongated beam path, means at the terminal end of the beam path for collecting and dissipating the energy of the beam, slow wave circuit means for electromagnetic interaction with the beam, said slow wave circuit including a plurality of cavity resonators successively arranged along the beam path, negative mutual inductive coupling means communicating between said adjacent cavity resonators, the walls defining said cavities including a plurality of conductive nose portions protruding into each of said cavity resonators, mode suppression cavity resonator means coupled to the fields of said first mentioned coupled cavities of said slow wave circuit for suppressing undesired modes of oscillation associated with said slow wave circuit, THE IMPROVEMENT COMPRISING, slot coupling means in said nose portions of said slow wave circuit for coupling said mode suppression cavities to the fields of undesired modes of oscillation within said slow wave circuit, and wherein at least a portion of each of said mode suppression cavities is disposed within said nose portions of said slow wave circuit.
 2. The apparatus of claim 1 wherein said slot coupling means in said nose portions includes a pair of coupling slots on opposite sides of each nose portion, said pair of slots being elongated in a direction generally perpendicular to the axis of the electron beam.
 3. The apparatus of claim 1 including a lumped capacitive loading means in each of said mode suppression cavities for capacitively loading said mode suppression cavities to reduce the physical size thereof for a given resonant frequency.
 4. The apparatus of claim 1 wherein said mode suppression cavities are tuned for resonance near the pi mode resonant frequency of said slow wave circuit for suppressing undesired upper band edge oscillations of said slow wave circuit.
 5. The apparatus of claim 4 wherein there are four mode suppression cavities coupled to each of said first mentioned coupled cavities of said slow wave circuit, and wherein each of said four mode suppression cavities is tuned to a different frequency within a band of frequencies centered near the pi mode resonant frequency of said slow wave circuit and above the operating band of said slow wave circuit.
 6. The apparatus of claim 1 including wave energy attenuating coating means coating the interior surfaces of each of said mode suppression cavities for attentuating wave energy coupled into said mode suppression cavities. 